Question

What are aromatic hydrocarbons?

Answer 1

An aromatic hydrocarbon is a hydrocarbon that contains `4n+2` electrons in a planar conjugated π system (`n = 0, 1, 2, …`).

The most common aromatic compounds contain benzene rings (`n = 1`).

Examples are toluene and trinitrotoluene (TNT).

But any planar cyclic hydrocarbon that contains 2, 6, 10, 14 … π electrons is aromatic.

Examples are naphthalene (10 π electrons) and phenanthrene (14 π electrons).

The ring doesn't have to contain a 6-membered ring, as long as it is planar contains `4n+2` π electrons

Thus, [18]annulene is aromatic. It is planar and possesses `4n+2` π electrons (with `n = 4`).